Summary

5.1 Viruses

• Viruses are generally ultramicroscopic, typically from 20 nm to 900 nm in length. Some large viruses have been found.
• Virions are acellular and consist of a nucleic acid, DNA or RNA, but not both, surrounded by a protein capsid. There may also be a phospholipid membrane surrounding the capsid.
• Viruses are obligate intracellular parasites.
• Viruses are known to infect various types of cells found in plants, animals, fungi, protists, bacteria, and archaea. Viruses typically have limited host ranges and infect specific cell types.
• Viruses may have helical, polyhedral, or complex shapes.
• Classification of viruses is based on morphology, type of nucleic acid, host range, cell specificity, and enzymes carried within the virion.
• Like other diseases, viral diseases are classified using ICD codes.

5.2 The Viral Life Cycle

• Many viruses target specific hosts or tissues. Some may have more than one host.
• Many viruses follow several stages to infect host cells. These stages include attachment, penetration, uncoating, biosynthesis, maturation, and release.
• Animal viruses can undergo latency, similar to lysogeny for a bacteriophage.

5.3 Prions

• Other acellular agents such as prions also cause diseases. Prions are proteinaceous infectious particles that cause transmissible spongiform encephalopathies.
• Prions are extremely resistant to chemicals, heat, and radiation.
• There are no treatments for prion infection.